THE SHAPES OF TEKTITES

INTRODUCTION

In this chapter we shall study the shapes of tektites,
beginning with those whose form is best understood, and working back toward earlier
and more primitive forms.

Definitions

Tektites occur in four general classes of forms:

Microtektites (see Plate 3), usually less than 1 mm in
diameter, and found, so far, only in ocean-bottom cores, but clearly
associated, both in composition and geographic location, with other tektites.

Splash-form tektites (see Plates 5A, B and 6).These form the great majority of all known
tektites.They look like congealed drops
of some viscous liquid; they are shaped like spheres, drops, dumbbells,
hamburgers, etc.They are typically
decorated with corrosion markings of various kinds:cupules (hemispheric pits of sizes up to
about 1 mm; see Plate 7A); gouges (elongated depressions, with sharp edges,
U-shaped in cross section, and with a length which is several times the width;
see Plate 7B); meandrine grooves (U-shaped in cross section, and in plan like
worm tracks in old wood; see Plate 7C, D).Sometimes, especially in tektites from Anda (see Plate 7E) in the
Philippines, the grooves show an astounding appearance of multiplicity, as if
they had been dug out by the claw of an animal.

Flanged buttons (see Plate 8A), and related forms; these are
found principally in Australia.The
central part, or core, is typically lens-shaped.On one side there are often concentric rings,
spaced a few millimeters apart, called ringwaves; these may also take the form
of a double spiral.The side with the
ringwaves is called the anterior side; it was almost certainly in front while
the tektite was coming down through the atmosphere.The opposite, or posterior, side often has
indications of corrosion, though usually not as strong as on the splash-form
tektites.Around the edge there is, in
some well-preserved specimens, a flange formed by glass dragged off the
anterior surface presumably by the airstream.

The splash-form tektites and the flanged buttons, when
sectioned, show a system of irregularly wandering striae (see Plate 8B), in the
interior, of varying color, hardness and composition.

Plate 8. Australite structures.A. Flanged australite. Courtesy of British Museum.B. Striae in slice of flanged australite. The striae meet the posterior surface (the upward surface) at a sharp angle; but at the anterior surface, they turn and follow the surface, as a result of liquid flow. Courtesy of D.R. Chapman.

Many tektites have been subjected to a process of
spallation, i.e. the breaking-off of a more or less flattened piece from the
outside, apparently as a result of thermal shock.Occasionally, the spallation is incomplete:a portion of the spalled surface adheres, so
that it is possible to be sure that this is how the remainder of the surface
was lost (Chapman, 1964).

THE FLANGED BUTTONS AND RELATED FORMS

Observations

The best-understood shapes among the tektites are the forms
of the flanged buttons (Plate 8A).Flanged tektites are almost unknown outside Australia; and the
well-formed flanged buttons occur chiefly in southeastern Australia.

The understanding of these forms was developed in the papers
of Stelzner (1893a, b), Fenner (1934, 1935, 1938, 1940a, 1949), Baker (1940,
1944, 1955b, 1958a, b, 1959a, b, 1960a, b, 1963c, 1967) and Baker and Forster
(1943).The evolutionary sequence as
finally worked out by Baker is indicated in Fig. 16.The tektite enters the atmosphere as a
relatively smooth glass ball. Glass is
first lost from the anterior surface; of this glass, the portion in the
equatorial zone is lost, at least in part, by spalling.When half or more of the central diameter has
been lost, it seems to become possible for glass to accumulate on the lee side
of the tektite, to form the flanged button.

Figure 16. The formation of flanged australites. After Baker, 1965b (Memoir of the National Museum of Victoria, Melbourne, No. 20, p. 91). A-H. Successive stages in the ablation of a glass sphere (A) entering the earth's atmosphere. Note that the flanges do not appear until stage E, when over half the mass is lost; and they break off for the last stages (G, H).

Experiment

Chapman et al. (1962) made some experimental studies in wind
tunnels which showed that in fact an airstream is capable of producing the
observed effects on tektite glass.Since
the Mach 15 to 25 velocity of a tektite with respect to the air cannot be
simulated in a wind tunnel, resort was had, as usual, to an arc jet, in which
the air is strongly heated by an electric arc, and is then driven toward the
model at about Mach 3.The total energy
content of the stream (internal energy plus kinetic and mechanical energy) is
the same as that in the actual case; and the fact that the velocity is
different does not matter because behind the shock the air velocities are
always subsonic in any case.

Plate 9. Three australites, diameters 16-26 mm, and three artificial models made from tektite glass ablated in the NASA-Ames arc jet. Courtesy of D.R. Chapman.

Chapman's remarkable australite-like models (see Plate 9,
upper line) were not produced starting from spherical bodies; instead, his
models in these cases started from lens-shaped pieces of tektite glass.Hawkins (1963), working with AVCO windtunnels,
attempted to produce the flanges experimentally by starting with spheres; he
was not successful.In his experiments
the melt flow went around to the back of the sphere; the results seem to have
resembled some javanites described by Von Koenigswald (1963b) in which an outer
layer, apparently of melt glass, forms festoons on the posterior surface.Among the australites these objects are
called crinkly tops by Fenner.It is
known from Fenner's studies (1938) that flanges are not found attached to
tektites until the main body of the tektites has been reduced to a lenticular
form; in addition, Hawkins gives aerodynamic reasons for expecting that in his
wind tunnel the flange glass will be lost, while in Chapman's somewhat faster
airstream, it will stay on.

Calculations

It is possible to calculate the ablation phenomenon, using a
theory developed for heat shields of artificial satellites and missiles.The plan of the calculations is to account in
detail for the heat dissipated as the tektite moved through the air.A mass mof air encountered by the tektite moving at a velocity V as seen by the
tektite, contains an energy ½ mV², plus relatively unimportant quantities or
energy which it contains as a result of the initial temperature.Of this energy, a calculable fraction is
dissipated at the shock; this is radiated away.Back of the shock is a layer of air a millimeter or so deep called the
gas cap.In contrast to the ambient
atmosphere, which is cold, thin, and moving at hypersonic speed past the
tektite, the gas cap consists of hot, dense air, which moves relatively
sluggishly, at subsonic speed, over the tektite surface.Most of the energy of the airstream has been
converted at the shock from kinetic energy of motion to thermal energy.As the gas cap flows over the tektite
surface, it carries away with it most of the energy of the airstream, which
then disappears into the wake.

The gas cap is at a temperature on the order of 7000°
K.Between it and the tektite surface
there is a very thin layer of air, the boundary layer, whose base is at the
temperature of the tektite surface, and whose top is at the temperature of the
gas cap.It is through this layer that
most of the heat comes which the tektite must cope with.This heat is denoted qaero.A small amount of qaerois radiated back into
the air; another small amount is absorbed in heating the body of the tektite.But most of the heat goes into melting and
vaporizing a thin layer of glass a tenth of a millimeter thick or less.The critical question concerns the balance
between melting and vaporization.

If the layer is thick, then the forces of skin drag will pull
it away from the stagnation point; it accumulates in the lee of the tektite
edge, as the flange.

If the layer is thin, then viscous forces, which depend on
velocity gradients, are able to hold the liquid in place until the heat
supplied is enough to vaporize it.In
the first case, about 500-600 kcal/kg are consumed; in the second case, around
3000 kcal/kg are used up.

A first attempt at a calculation of the flow was made by
O'Keefe (1960); this was immediately superseded by the work of Chapman (1960). Adams and Huffaker (1962, 1964) and Chapman
and Larson (1963) produced detailed mathematical programs which gave the
calculated ablation as a function of entry angle and entry velocity.Of these calculations, those of Chapman and
Larson indicated that most of the ablation in tektites always occurs as the
result of melt flow; in their calculations, this always predominates over
vaporization by a large factor.For 1-cm
tektites entering at 11 km/sec the ablation ranges from 8 to 15 mm, depending
on the entry angle, and being largest for the shallowest angles of entry.The calculations of Adams and Huffaker
(1964), on the other hand, were based on a vapor pressure for tektites which
was much greater than that accepted by Chapman and Larson.Adams and Huffaker found large ablation and
melt flow only for angles which nearly graze the upper atmosphere (skipping
trajectories, in particular, which actually climb out of the atmosphere and
then fall back); moreover, they found that vaporization can remove as much as
60% of the tektite.Their use of a high
vapor pressure was attacked by Centolanzi and Chapman (1966) who demonstrated
that the vapor pressures of Walter and Carron (1964) – which were used by Adams
and Huffaker – referred to the most volatile constituents of the tektite only
(for example, water); and that the vapor pressure corresponding to the majority
of the matter of the tektite was not far from that of pure silica, as Chapman
and Larson had assumed.The accuracy of
the calculations of Chapman and Larson was confirmed by O'Keefe et al. (1973)
using the theories of Adams and of Warmbrod (1966; see Fig. 17).

The vitally important fact, on which all these calculations
agree, is that the amount of ablation observed on australites corresponds to
velocities coming from space (i.e. on the order of 11 km/sec).It is much too great for the kind of
velocities to be expected for tektites which are following ballistic
trajectories from one point to another on the earth (see Table II); these range
up to about 6 km/sec.As we shall see,
it is possible to understand how tektites moving at 11 km/sec might suffer much
less ablation than that of Fig. 17; what is not comprehensible is how tektites
moving at 6 km/sec or less could suffer as much ablation.

SPLASH-FORM TEKTITES:THE CORROSION PROBLEM

Thus a difficulty has arisen in understanding tektite
ablation.The aerodynamic calculations
indicate that nearly all ablation is by melt flow, and ablation is always a
matter of the order of a centimeter, and hence sufficient to change the overall
shape of a tektite of typical size in a significant way.In fact, however, splash-form tektites almost
never show evidence of melt flow; the only exceptions are some javanites (Von
Koenigswald, 1963b; see Plate 10).Moreover, the shapes of a large class of tektites are those of liquid
drops:spheres, dumbbells, etc., without
the changes of overall shape which is observed in australites and is predicted
by the theories of Chapman and his co-workers.Chapman explains the absence of evidence of melt flow by saying that the
outer surface, with the marks of aerodynamic ablation on it, has been removed
by spallation, followed by etching by ground chemicals.There is no doubt that spallation does
account for important features, particularly in the sequence from australitesto Philippine tektites (Plate 11).The meandrine grooves seen on the lower
(anterior) side of these tektites are probably an indication that thermal shock
has taken place; hence spallation is plausible.

In general, however, spallation does not account for the
absence of visible melt flow, hence the question of the amount of corrosion by
ground chemicals is a critical point in understanding the aerodynamic effects
on tektites.Many authors consider that
attack by ground chemicals may have destroyed the flanges and the ringwaves,
and may have produced the system of pits and grooves.The abundant evidence on this point is widely
scattered through the literature; it will be summarized in the next section.

Plate 10. Flange formation on a javanite. Note the thickness of the melt layer, compared with that on the australite (Plate 8B). Courtesy of G.H.R. von Koenigswald.

Causes of tektite corrosion

Let us adopt the term corrosion (Suess, 1900, p. 256) for
the process by which many tektites acquired their characteristic sculpture of
grooves, pits, notches, and gouges.In
using this term we do not intend to imply anything about the origin of the
corrosion, whether aerodynamic or chemical or from any other cause.

Corrosion on australites.A typical australite button has three different kinds of surface:the anterior surface, the rear surface of the
flange, and the true posterior surface (seen bulging upward within the ring of
the flange).It will be shown that
chemical attack has been negligible in shaping each of these surfaces.

Clearly there has been little attack on the anterior
surfaces of the flanged buttons.Corrosion is normally not shown at all; occasionally there are a few
small hemispherical pits.The maximum
amount of loss by ground chemicals can be estimated from the curvature of some
striae which are observable in the glass.The striae are due to variations in the composition of the glass; they
form a complex system of thin-folded structures inside the tektite, like a
crumpled wad of paper.As the striae
approach the anterior surface, they turn aside and run parallel to it (see
Plate 8B); this is an obvious result of the flow of the glass, under the
influence of aerodynamic drag, away from the center of the anterior surface and
around toward the sides and back.By
fitting theoretical curves to the observed striae, Chapman et al. (1962, p. 14)
have estimated that only about 0.12 mm was removed after the tektites stopped
ablating.

Like the anterior surfaces, the posterior surfaces of the
flanges show very little evidence of corrosion.Study of the flanges has not been made in the same way as on the
anterior surface, but it is reasonably clear from photographs of sections of
flange glass that there has been little chemical loss.

On the posterior surface of the main body of the australite
– which is usually referred to as the posterior surface – there is often
corrosion, particularly in the form of small hemispherical pits, and it often
happens that the striae are seen standing out in relief.McColl (1966) notes that this resembles the
corrosion seen in other tektites.It is
important to decide whether this corrosion is due to ground chemicals or was
already present when the tektite fell.

The difference between the anterior and the posterior
surfaces of the australites cannot be somehow a result of their position in the
ground (e.g. the lower surface being differently attacked from the upper
surface) because the flange glass shows no corrosion in many cases when the
posterior surface of the main body is corroded.

It is reasonably certain that at least some of the corrosion
on the posterior surface is produced before the tektite strikes the ground,
because it has been found under the flange glass.Dunn (1912) illustrates this point by
photographs of flanged australites which have been sliced perpendicular to the
axis of symmetry, right through the region where the flange joins the core. Baker (1944) noted pits under the flanges,
sometimes infilled with flange glass.Barnes (1962b) also noted hemispheric pits, which he attributed to
bubbles, filled with flange glass.Baker
(1963a) concluded that the sculpture of the posterior surface is
pre-atmospheric.

On the other hand, it is clear from other thin sections made
in the same way that the posterior surface of the australite is sometimes much
rougher outside the flange than under it.This roughness is on a scale of a few tenths of a millimeter; and it
follows the striae.Clearly, this attack
occurred after the tektite reached the earth.Clearly, also, it is entirely different from the sculpture of the
splash-form tektites, since it follows the striae, while the typical sculpture
of splash-form tektites does not.

The lack of attack by ground chemicals on Australian
tektites would of itself suggest that attack by ground chemicals is very slow,
except for the unfortunate fact that, as mentioned in Chapter 2, the date of
arrival on earth of the Australian tektites, which include almost all of the
flanged tektites, is controversial.In
that chapter, however, strong arguments were put forward indicating that the
australites belong to the Australasian strewn field.It follows that the absence of terrestrial
etching on australites (more exactly, the very low level of etching) is
evidence that the rate of attack has been small.This evidence is significant not only for the
very arid areas of Australia, but even for the more humid areas; Baker (1960b)
goes so far as to say that the best-preserved australites are from humid areas.

Corrosion on splash-form tektites.Von Koenigswald (1963a) points out that
flight markings, melt flow and flanges also occur on some javanites (Plate 10)
(though no flanged buttons); in this case the tektites are found associated
with mid-Pleistocene fauna (Von Koenigswald, 1958):Homo erectus and a primitive elephant-like
animal Stegodon.The presence of the
melt flow suggests that the Australian tektites are only an extreme case of
general trends across the Australasian strewn field, and hence should have the
same age as the others; these are dated by by K-Ar and fission track, and also
by standard geological methods, at -700,000 years.

The evidence for melt flow consists of rolled-up flanges,
like those on australites, flow ridges, like the concentric flow ridges of
australites, and in many cases an anterior surface whose curvature is markedly
less than that of the tektite as a whole. The ablated surfaces are free of
corrosion; yet in general the javanites belong to the family of splash-form
tektites and are corroded (Von Koenigswald, 1964).

Among the javanites, Von Koenigswald (1961b) has drawn
attention to a particular hollow specimen which appears to have been plastic
when a pit was inflicted on it.The
tektite was afterward broken open by natural causes, and it is possible to see
the inside of the tektite; there is a lump on the inside just back of the pit
on the outside, as if the pit had resulted from some inward-acting force while
the tektite was still plastic.

Beyond Java and Indonesia, in the South China Sea, between
the Philippines and Indochina, four tektites were brought up by a dredge haul
from the bottom of the sea (Saurin and Milliès-Lacroix, 1961) whose exterior
sculpture differed little from that of land tektites.These were examined by Barnes (1971a) who
found attached nannofossils which could be dated at -1.0 to -1.3 million years.Since the sculpturing must have been complete
before the nannofossils were attached, Barnes concluded that the rate of attack
must have been much more rapid in the earlier part of the life of these
tektites on earth. The evidence is obviously more easily explained as a result
of sculpturing before arrival at the earth.

If in fact the sea tektites were etched by seawater, then it
is difficult to understand the existence of the microtektites.There are many of these in the size range
around 40 μm.Since the total amount of
corrosion is typically on the order of 1 or 2 mm, it is clear that if this
corrosion had persisted for a time only 2% greater, all of the smaller
microtektites would have disappeared completely.It is also difficult to understand why
tektites whose initial radius was, let us say, 1.02 mm should have been so much
more abundant than those whose initial radius was 1.05 mm; yet if etching had
removed 1 mm, the first set would have become the present 40-μm spheres, and
the second the present 100-μm spheres, which are much less abundant.It is much more likely that the etching has
been essentially zero in the sea, and therefore that the sea tektites were
already corroded when they arrived.This
conclusion is strengthened by the recent discovery (Glass et al., 1972a) of
microtektites in the Caribbean, which have survived for 35 million years under
the sea, or about 50 times longer than the Australasian microtektites.

A large number of tektites have been found on the Indonesian
island of Billiton, in the course of tin-mining operations.These were examined by Easton (1921) who
pointed out that while many of them have the forms of complete droplets
(spheres, pears, etc.); there are also many forms which appear to have
broken.He found that the typical
tektite sculpture never appears on the broken surfaces. This could be understood only if the break
occurred after the period of sculpturing.Yet the broken surface was not truly fresh, as it would be if the miners
themselves had broken it.

The absence or scarcity of sculpture on broken surfaces was
also noted by Suess (1900, pp. 257-258) on moldavites; by Van der Veen (1923)
on billitonites; by Lacroix (1929, 1932) on indochinites; by Von Koenigswald
(1961b) on javanites (as well as billitonites); by Barnes (1939, p. 503) on
North American tektites; by Rost (1969) and Žebera (1968) again for
moldavites.Rost notes that when two
fragments of the same tektite are found separately and are reunited, it can be
seen that the common surface is only slightly etched.Kaspar (1938) notes that when moldavites are
broken, exposing the interior of an ancient bubble, the bubble surface is never
corroded.Lacroix (1930) notes the same
for indochinites (Plate 2E, F); it is shown on a thailandite (see Plate 12A, B,
C) and on a lei-gong-mo (Plate 12D, E).It is incredible that in every case the tektite could have been broken
only a short time before it was found.

The evidence on the moldavites is particularly interesting;
some of these have obviously been worn by stream erosion.Žebera (1968) notes that even when the stream
action can be dated, and took place millions of years ago (Pliocene), there is
no sculpture formed on the worn surfaces.Baker (1937) noted that in a collection of 83 tektite fragments only two
could be put together; he concluded that fragmentation took place in flight.

Nininger and Huss (1967) found two indochinites (see Plate
13A, B) which appeared to have suffered incomplete breaks while still in a
plastic condition.The surfaces of these
tektites are covered with the usual decorations, except where the plastic break
exposed new surfaces.There is a clear
implication that these tektites broke while still in a plastic condition, but
after the completion of the sculpturing.Since the tektites could scarcely have been made plastic after they
reached the ground, this appears to be evidence that these tektites at least
were already corroded when they struck the ground.

Suess (1900) made a fundamental contribution tothis problem in a monograph on the
Czechoslovakian moldavites.He found
that the markings on the moldavites are arranged in patterns which depend on
the overall shape of the tektite.This result
is understandable if the patterns are produced by vortices and shock waves in
hot gases, since these must satisfy differential equations of fluid flow which
involve the shape of the specimen.The
result does not make sense if the patterns were produced by the blind action of
underground chemicals or plant roots.

For example, Suess found that if the tektite has the general
shape of an oblate spheroid (like a Gouda cheese) then a system of gouges is
often found, which radiate from the center of one face (see Plate 14A, B).On very flattened spheroids (watch-shaped
bodies) there is often a set of gouges which go across the rim in a direction
perpendicular to the equator of the body.On convex surfaces, the gouges tend to run in the direction of greatest
curvature, while on concave surfaces the gouges run in the direction of least
curvature.A hemispherical pit is often
found to be the center of a star-like configuration of radial gouges.When a surface contains mostly hemispherical
pits, the gouges are not seen (Plate 7A).When a surface has pits on one part, and gouges on another, the regime
of pits appears to precede the regime of the gouges.

Somewhat similar rules were found to hold for billitonites
by Easton (1921); navel-like depressions occur on the most strongly curved
surfaces, for example.

Suess also found that the sculpture of moldavites is not
usually related to variations in chemical composition.The surfaces of tektites generally show some
faint swirling marks (see Plate 14C, D) which look like what you would see if
you cut through a crumpled and folded stack of pancakes.These lines are called the streaky
structure.The streaks are related to
the chemical composition of the tektite; streaks with more silica tend to stand
out very slightly.They correspond to
the striae (Plate 8B); they are the lines where the striae come to the
surface.The important point is now that
the streaky structure may run at any angle to the gouges of the main tektite
structure.In a detailed piece by piece
description of some 43 specimens, Suess (1900) makes this point again and again.

If, on the other hand, a tektite is put into a weak solution
of hydrofluoric acid, so that it is slowly etched away, then Suess found that
this artificial attack follows the lines of the streaky structure.One would therefore expect that if the sculpture
is due to ground acid etching, it would also follow the lines of the streaky
structure; but the major sculpture does not.

When tektites have been found chipped by primitive man, it
is always found that the chipped surfaces are uncorroded.For moldavites found at Willendorf (Plate 1)
this is noted by Suess (1914).Beyer
(1934b, p. 106) reports the same for late Paleolithic artifacts made from
Philippine tektites.Baker (1962) finds
a similar result for Australian chipped tektites (Plate 2A, B).

When glass is attacked by ground chemicals, a residue of
highly silicic material is left behind (Rzehak, 1912b; Brill, 1961).This residue has been observed on obsidians
(Wright, 1915); it is in fact used for purposes of dating. Baker (1961) found
that etching with citric acid, a normal soil constituent, produced a white
crust.It is not produced by
hydrofluoric acid.But no such residue
has ever been noted on a tektite when found.

In favor of the origin of tektite sculpturing by ground
chemical activity is the fact noted by Berwerth (1910) to the effect that
tektite sculpture is not really like the sculpture of meteorites; and that on
meteorites the effect of atmospheric ablation tends to round the bodies, rather
than to roughen them.Linck (1928) used
similar arguments.Diaconis and Johnson
(1964) attempted to produce the typical tektite sculpture by using artificially
heated tektites, since this can tend to make the boundary layer turbulent.Once again, it turned out that the effect of
aerodynamic ablation, when entry into the earth's atmosphere is simulated, is
to smooth the specimens rather than to produce the characteristic tektite sculpture.Only when air of much higher density was used
did the sculpture appear (Golden and Blackledge, 1968).This result can be understood if the
sculpture is not atmospheric but is due to envelopment in some gas such as that
which launched the tektite.Linck (1928)
suggested that the tektites had been launched from a lunar volcano, and that
the sculpture was the mark of these volcanic gases.For the moment, the point to see is that this
objection, namely that the earth's atmosphere will not do the trick, does not
necessarily imply that the sculpturing is produced by ground chemicals.

Van der Veen (1923) removed all of the existing sculpture
from some tektites, then heated them and quenched them in a water jet.He found that a set of cracks developed on
the outer surface.If the tektite is then
attacked by HF, the attack follows the pattern of the cracks, and the resulting
pattern of grooves is very much like the meandrine grooves seen especially on
some billitonites (Plate 7C, D).Like
the natural grooves, the grooves so produced were found to have a U-shaped
cross section.These results were
confirmed by D.R. Chapman and F.J. Centolanzi in some unpublished work which
they kindly communicated to me (1973; see also Chapman et al., 1967b). In this
case, the resemblance to tektite sculpture is really convincing.It seems possible that some kind of attack
really can occur along such cracks.A
possibility that does not seem to have been excluded is that during the formation
of the crack itself, powerful local stresses produced a mesh of small cracks
around the main crack.Later the chips
fell out, or were dissolved out by some very short-range action.A similar explanation might apply to the
deepening of the crack between the flange and the core, which can be seen on
some australites; the flange glass is at a different temperature from the core
glass when they are welded together.Glass (1974) has found that when two microtektites are welded together,
a groove is found along th line of contact.In this case, however, the groove is found to be V-shaped in cross
section.

Baker (1963c) considers that the sculpturing must be due to
ground chemicals because he feels that the Australian flanged buttons are very
young.The buttons are clearly not
corroded on the anterior surfaces, and he feels that these two facts are
connected.The argument evidently falls
to the ground if australites have the same age as the other Australasian
tektites.Curiously, Baker regards the
sculpture of the posterior surface of flanged australites as
preterrestrial.He notes (1963b) the
mixture of corroded and uncorroded tektites side by side in Western Australia
at Nurrabiel.He finds (1961) that acid
attack is closely related to the striae (which is not usually true of the corrosion).

Summary of arguments.Summing up, the arguments in favor of the terrestrial origin of the
tektite sculpture are:

(1) The absence of sculpture on australites, especially the
anterior surfaces, combined with 14C evidence for the low ages of the
australites.

(1)Evidence that the
australite flanged buttons have been on earth as long as other Australasian
tektites; this evidence comes from fission tracks, K-Ar dating, paleomagnetics
(for the microtektites), and the close chemical ties of australite groups to
other Australasian tektite families.If
the high age is accepted, then the lack of pits and gouges on the anterior
surfaces of australite flanged buttons and the evidence for low (0.12 mm) loss
of surface glass becomes evidence pointing against the terrestrial origin of the
tektite sculpture.

(2)Sculpturing,
especially pits,on the posterior surface of australites, especially that under
the flange glass, which is qualitatively similar to the sculpture on the
splash-form tektites.

(3)Pits and other
sculpture on some surfaces of javanites, with australite-like ringwaves on
other parts of the same tektites.Flanges on tektites directly associated with Pleistocene fossils.

(4)Existence of
microtektites, many less than 40 µm in diameter, for periods up to 35 million
years in seawater.Some Australasian
tektites with sculpture are found in waters which also have microtektites
700,000 years old.

(5) Absence of sculpture on broken surfaces, on the interior
of broken bubbles, and on surfaces formed by plastic breaks.

(6) Correlation of pits and gouges with overall shape; lack
of correlation with compositional variations.

(7) Lack of the siliceous crust usually formed by glass
decomposition.

The arguments against the terrestrial origin of the
sculpture appear overwhelming.The
arguments for terrestrial origin can be met if:

(1) The australites have the same age as other Australasian
tektites.

(2)The meandrine
grooves are due to thermal cracks enlarged by some mechanism other than
chemical attack.Note that the Bikol
(Coco Grove) tektites (Plate 15A, B; Beyer, 1938, part 2, p. 143) were dredged
from the sea bottom, yet have enlarged meandrine grooves.

(3) The sculpture is produced, not during entry into the
earth's atmosphere, but (except for the meandrine grooves) in some earlier
phase.

It is therefore concluded that most tektite corrosion cannot
be due to action by ground chemicals; that it was already present on the
tektites when they reached the earth's surface.

ABLATION OF SPLASH-FORM TEKTITES

It follows that we cannot explain the absence of the marks
of aerodynamic ablation on tektites by appealing to corrosion by ground
chemicals. Unless these marks have been removed by breakage (e.g. spalling),
the tektite must be carrying the marks of aerodynamic ablation.The role of spalling in removing the
aerothermal stress shell, as Chapman (1964) calls the portion of the tektite
stressed by aerodynamic heating, cannot be denied.It is clear, however, that every unbroken
tektite form must carry either spall marks or a place where the effects of
ablation are visible.

In the case of the flanged buttons, the results of
aerodynamic ablation are clear; but what about the much commoner splash-form
tektites?Should we follow Suess in
regarding the sculpturing itself as the result of downward passage through the
earth's atmosphere?

Probably not, for the following reasons:

First, as mentioned above, this kind of sculpture, though it
may result from gas flow, seems to require turbulent flow of a relatively dense
gas.It does not seem to be possible to
explain the attack by dense gases of the kind required here under conditions
which simulate tektite entry; at least, all efforts to do so have failed so
far.

Second, as pointed out by Berwerth (1910) and others, the
sculpture observed on meteorites is not really like that on tektites; by
comparison, meteorite sculpture seems to smooth the surface, at least if we are
thinking on a scale of millimeters.

Third, some of the sculpture on the posterior surface of
australites seems to predate the formation of the flanges, as noted above.Conceivably it could represent an earlier
stage of aerodynamic ablation; but this is improbable because higher atmospheric
density favors turbulent flow over laminar flow.The flanges are the result of laminar flow;
if there is to be turbulent flow in descending flight, it should come after the
laminar flow; but a small amount of corrosion is observed under the flanges.

Fourth, the sculpture on the Nininger and Huss (1967)
specimens seems to have been put on while the tektite was still plastic.Watson (1935) pointed out the difficulty of
heating a mass the size of a tektite all the way through during a meteoric
passage through the atmosphere.It would
require a nearly grazing approach; and would also, probably, mean that the
tektites would have to form by the sweeping-off of a liquid layer (O'Keefe,
1963).This idea has had to be given up
(O'Keefe, 1969a) because it cannot be made to fit the microtektite data.Hence the Nininger specimens point to
sculpturing at the source.

Similarly, Bouška (1972) found pairs of moldavites in which
one had plunged into the other while the specimens were still plastic.The pattern of corrosion is different on the
two pieces as if already established when they were joined.

If the Nininger and Huss specimens are examined, it will be
seen that in addition to the plastic breaks, they have considerable areas which
are bare of all corrosion (Plate 13B).These areas may be called bald spots.Is it possible that these bald spots represent the results of
aerodynamic ablation?

If we examine the splash-form tektites with this question in
mind, we will note that a very large number of them do have a sort of bald
spot, where the sculpture seems to have worn away (Plates 12C, 14D and 15C).In museum specimens, this is usually the
place where the curator puts the label on.It does not, of course, always appear on broken tektites.On unbroken tektites, the spot is often more
easily detected by the sense of touch than by sight; but it is almost always
there.In the few cases when it cannot
be found, the reason may be that the tektite tumbled in flight, so that the
ablation was evenly distributed over the whole object.

D.R. Chapman (personal communication, 1973) argues that
these bald spots result from spallation; and there can be no question about the
fact that this is sometimes true.On the
other hand, there also appear to be cases when the bald spot includes some pits
in it, as if these had been too deep to be scrubbed off.When these surviving pits are numerous, it
becomes evident that the spalled fragment, if any, would have been lacy with
holes.It does not seem mechanically
plausible that such an object would break off in one piece.

The main point is, however, that ablation on the splash-form
tektites must have been much less than that on the australites.This is not a matter of minor details, but of
overall form.Where there are spheres
among the indochinites, there are lenses among the australites; where there are
drops or dumbbells or rods, the australites have the equivalent form but
flattened.The flattening is clearly not
a matter of deformation while in a plastic condition; it is rather a matter of
the loss of some material.There are
some tektites outside Australia whose overall form simulates that of the
australites (King, 1964a; Chao et al., 1964b; Von Koenigswald, 1967; Soukenik,
1971b); but they are rare.The general
rule is that the overall shapes of the splash-form tektites resemble the shapes
which the australites must have had before they were ablated.

We seem to be driven to suppose that the splash-form
tektites suffered some kind of ablation (connected with the bald spots) but
that this ablation was quantitatively much less than that of the australites.

Furthermore, it appears that the ablation of the splash-form
tektites occurred without melt flow, or with only very minor melt flow, as in
the case of some rare javanites.

Is it possible to imagine circumstances such that in a
single event some of the infalling objects are deeply ablated, losing up to a
centimeter in depth, at least some of it by melt flow, while in the same fall
(but not in the same area) other objects lose only a millimeter or two in
depth, with very little of the loss being due to melt flow?Chen (1974) considers that the differences
may result from roughness in the preatmospheric shape; he investigated this
point in detail.

MUONG NONG TEKTITES; MICROTEKTITES

The Muong Nong-type tektites (Plate 4A, B) appear to be
chunks broken out of an extensive layered mass; their forms do not appear to
have any further significance.The
Libyan Desert glass (Plate 16A, B) belongs to this category (Barnes,
1963b).The Darwin glass (Plate 17A) and
the Aouelloul glass (Plate 17B) are also closely related to the Muong Nong
category (Barnes, 1963b); they show clear evidence of a layered structure, but
it is often contorted.

The overall forms of the microtektites resemble those of the
splash-form tektites.They are
presumably governed by the same considerations.

BODY SHAPES OF TEKTITES

Underlying the australite flanges, and the pits and gouges
of the splash-form tektites are the general body shapes.Most australites and many splash-form
tektites are spherical.Others are
oblate spheroids, rod-shaped bodies, dumbbells, tear drops, or canoes (see
Plates 5A, B and 6).

The spheres can obviously be thought of as large drops.For the shapes of the other bodies, Fenner
(1934) suggested that rotation had played a major role.This idea has been widely accepted (e.g.
Baker, 1959b); but according to an important paper by Tobak and Peterson (1964)
it is wrong.They remark that under
surface tension, a figure of equilibrium must be rotationally symmetrical
around the actual axis of rotation; this rules out both the dumbbell and the
(prolate) ellipsoid, because both bodies could rotate stably only around their
short axes. Tobak and Peterson deduce that the shapes of splash-form bodies
resulted from the breakup of a jet, which was turning slowly or not at all.

CONCLUSIONS

The most important conclusion is that the flanged
australites appear to have entered the atmosphere, as Chapman and his
co-workers have claimed, at velocities near 11 km/sec, and at angles to the
horizon on the order of 30°.

Australites seem to have entered the atmosphere as smooth
bodies, usually spherical (since the surfaces under the flanges are smooth).

Splash-form tektites seem to have entered the atmosphere as
rough bodies; the roughness, judging from the bent tektites of Nininger, was
impressed while the inside of the tektite was still hot and plastic.

It may be that the very different pattern of ablation found
on most splash-form tektites (some bald spots butno liquid flow) is connected with differences
in initial surface sculpture.

It does not seem likely that tektite sculpture was produced
by ground chemicals.

Plate 13. Casts of tektites of having plastic breaks. Courtesy of H.H. Nininger and G. Huss. A. View of plastic breaks. Note that the corrosion of the exterior surface must have occurred while the tektite was still hot and plastic.

Plate 13 (continued). B. Bald spot on the larger tektite shown in A. Possibly the result of aerodynamic ablation.

Plate 18. Tektite internal structure. A. Moldavite, about 3 cm in diameter, from the Smithsonian collection. Immersed in light machine oil, and viewed between crossed polaroids to show internal strain pattern. Pattern is explicable as due to cooling as a unit..B. Spinous voids in a Muong Nong tektite from Phaeng Dang. Loaned by V. Barnes.